Paired Activation of Two Components within Muscarinic M3 Receptor Dimers Is Required for Recruitment of β-Arrestin-1 to the Plasma Membrane

Abstract β-Arrestins regulate the functioning of G protein-coupled receptors in a variety of cellular processes including receptor-mediated endocytosis and activation of signaling molecules such as ERK. A key event in these processes is the G protein-coupled receptor-mediated recruitment of β-arrestins to the plasma membrane. However, despite extensive knowledge in this field, it is still disputable whether activation of signaling pathways via β-arrestin recruitment entails paired activation of receptor dimers. To address this question, we investigated the ability of different muscarinic receptor dimers to recruit β-arrestin-1 using both co-immunoprecipitation and fluorescence microscopy in COS-7 cells. Experimentally, we first made use of a mutated muscarinic M3 receptor, which is deleted in most of the third intracellular loop (M3-short). Although still capable of activating phospholipase C, this receptor loses almost completely the ability to recruit β-arrestin-1 following carbachol stimulation in COS-7 cells. Subsequently, M3-short was co-expressed with the M3 receptor. Under these conditions, the M3/M3-short heterodimer could not recruit β-arrestin-1 to the plasma membrane, even though the control M3/M3 homodimer could. We next tested the ability of chimeric adrenergic muscarinic α2/M3 and M3/α2 heterodimeric receptors to co-immunoprecipitate with β-arrestin-1 following stimulation with adrenergic and muscarinic agonists. β-Arrestin-1 co-immunoprecipitation could be induced only when carbachol or clonidine were given together and not when the two agonists were supplied separately. Finally, we tested the reciprocal influence that each receptor may exert on the M2/M3 heterodimer to recruit β-arrestin-1. Remarkably, we observed that M2/M3 heterodimers recruit significantly greater amounts of β-arrestin-1 than their respective M3/M3 or M2/M2 homodimers. Altogether, these findings provide strong evidence in favor of the view that binding of β-arrestin-1 to muscarinic M3 receptors requires paired stimulation of two receptor components within the same receptor dimer.